There are several factors that need to be considered when operating a municipal wastewater works and this includes laboratory testing requirements to assist in understanding the efficiency of the treatment process.

The decision on the choice of the wastewater treatment practice depends on the quality of the incoming raw water, the required quality of the final treated water and the economic resources available for the operating costs of the treatment plant.  The requirements for laboratory testing play a significant role in the operation and maintenance of the plant.

Both compliance and process samples are taken throughout the works at specific sampling points which are essential to trend the process efficiencies and evaluate the compliance of the plant to general effluent standards.

Domestic wastewater treatment processes can be broadly classified in one of five categories as follows.

•  Physical separation of solids from the flow by screening and settling out heavy inert grit.
•  Primary treatment follows this pretreatment which is also a physical removal process.  Gravity settling in primary clarifiers removes some of the suspended organic material and most of the total suspended solids. 
•  Secondary treatment processes are biological processes that use bacteria and other microorganisms to breakdown the organic material that was not removed in primary clarification.  Secondary treatment processes include trickling filters and different kinds of activated sludge processes.
• Tertiary treatment processes follow secondary treatment.  Tertiary filter processes can be used to remove suspended solids.  Nutrient removal processes are also considered to be tertiary treatment processes. 
• The final process in wastewater is disinfection.  Chlorination is the most common means of disinfecting wastewater. 
• The treatment plant process control sampling comprises on-site tests carried out on a regular basis at the works.  These tests include pH, total dissolved solids, dissolved oxygen, conductivity, chlorine and half hour sludge settlements.

Further important process control considerations in secondary treatment are temperature, pressure and salinity which affect the saturation concentrations between the gas and liquid phase for both aerobic and anaerobic digestion.  Treatment plants in South Africa are moving away from the mundane manual operating systems of running the works to more automated processes.  Operational sensors and testing probes at key points throughout the treatment process allows for fine-tuning and conditioning of valves, aerators and dosing levels.  On-site testing equipment must also be encased due to the erratic nature of incoming load.  It is crucial that the equipment used on site is calibrated and monitored for any malfunctions as it plays a vital role in optimizing the works.

Compliance sampling at the wastewater works is undertaken by the laboratory using the utmost care to maintain the integrity of the samples during sampling and transportation to the laboratory.  Samples are taken at different stages in the wastewater treatment process to understand the operational efficiencies and compliance to the general effluent standards. 

When sampling for analysis of microbiological constituents, it is also very important to adhere to the aseptic sampling procedures.  The preservation period of the samples, the correct sampling containers, as well as the appropriate neutralizing agent in the sample bottle is imperative in the final determination of these bacteriological components. 

Sample sites for this compliance sampling phase includes but not limited to samples from the process points noted below.  The results of laboratory testing of the final effluent sample are used to determine compliance with the general effluent standards.
1. Incoming raw water
2. Settling tanks
3. Mixed liquor
4. Final Effluent

Laboratory testing in each instance above may comprise a combination of the following tests:
Ammonia, Chemical oxygen demand, Chloride, Conductivity, E. coli, Free chlorine, Heavy metals (where requested), Nitrate/Nitrite, Oil & Grease, Orthophosphate, Oxygen absorbed, pH at 25°C, Suspended solids at 105°C, Total alkalinity

Some of the analytical challenges facing the laboratory when undertaking testing of wastewater samples includes those highlighted below.
1. Constituents in the incoming raw water, such as oils, fats and inorganic substances that have passed through the screens in the treatment process; affect the laboratory analysis.  Sample preparation such as filtering of the samples to eliminate the coarse suspended or floating matter will facilitate a more accurate result.

2. During the tertiary stage of treatment, all factors that make up the effluent for discharge have to be considered.  Chlorine either in liquid, solid or the gaseous phase is used for final disinfection.  High concentrations of this disinfectant and oxidizing agent can cause matrix interference in certain test procedures.  For example chlorine obscures the end point titration values when testing for oxygen absorbed  as it masks the desired titration end point colour.

3. When conducting the chlorine test on-site using the conventional colorimetric method, chlorine in high concentrations inhibits the reagent from producing a distinct color change, as the reagent is oxidized almost immediately. Dilutions must be conducted in order to accomplish accurate results.

4. Analysis of elemental metals in wastewater using inductively coupled plasma mass spectrometry has made significant improvements in time management, cost efficiency and more so accuracy of results.  The ICP-MS is a type of mass spectrometry capable of detecting elemental metals and several non-metals at concentrations as low as one part per trillion (ppt).  Compared to atomic absorption techniques, ICP-MS has greater speed, precision, and sensitivity.  In addition, the sample preparation is relatively simple and quick.  There are many aspects of maintenance that need to be incorporated by daily, weekly and annual procedures where the frequency of maintenance is typically determined by the sample volumes and increasing run time that the instrument is subjected to.

Flow injection analysis offers many advantages over traditional wet chemistry testing for the determination of nutrients.  The procedure is inexpensive and quick, often producing results within very reasonable turnaround times.  The equipment is computer compatible, providing quicker analysis while limiting data entry errors more prevalent in manual testing. 

Constraints in these automated methods include the sample matrix and samples with an elevated conductivity.  This results in additional sample preparation that may include but not limited to sample pre-treatment or dilution before analysis.

5. The COD test is commonly used to indirectly measure the concentration of organic compounds in water, making the test a useful measure of water quality in wastewater management.  Some samples contain high levels of oxidizable inorganic materials which may interfere with the determination of COD.  Chloride is often the most prevalent source of interference. 
To overcome this challenge, the sample is treated with mercuric sulphate prior to the addition of other reagents to eliminate chloride interference.

These laboratory challenges are overcome with effective laboratory management through the implementation of a suitable quality management system that addresses the following:
• Administrative controls
• Staff competence
• Method development and validation
• Calibration of equipment
• Sampling and handling of the sample at the laboratory.

Talbot & Talbot offers scientifically engineered solutions that cater to the requirements of operating and managing wastewater treatment plants with the support of an accredited laboratory in the Talbot Laboratories division of the company.  The laboratory is accredited to the ISO 17025 standard for water and wastewater testing.